Well-defined nanostructuring with designable anodic aluminum oxide template

Well-defined nanostructuring over size, shape, spatial configuration, and multi-combination is a feasible concept to reach unique properties of nanostructure arrays, while satisfying such broad and stringent requirements with conventional techniques is challenging. Here, we report designable anodic aluminium oxide templates to address this challenge by achieving well-defined pore features within templates in terms of in-plane and out-of-plane shape, size, spatial configuration, and pore combination. The structural designability of template pores arises from designing of unequal aluminium anodization rates at different anodization voltages, and further relies on a systematic blueprint guiding pore diversification. Starting from the designable templates, we realize a series of nanostructures that inherit equal structural controllability relative to their template counterparts. Proof-of-concept applications based on such nanostructures demonstrate boosted performance. In light of the broad selectivity and high controllability, designable templates will provide a useful platform for well-defined nanostructuring.

The authors presented an interesting investigation of dynamic anodization voltage control induced complexity-diversified alumina pattern formation on the pre-pressed aluminum foil. The asdemonstrated work successfully overcomes the limited access of the current techniques in literature to in-plane and out-of-plane shape variation/control, realizing a remarkable level of complexity control over the AAO template fabrication process, which is indeed a breakthrough in the template based nanostructuring for well-defined nanofeature arrays of 3D heterogeneity, complexity, as well as periodicity. Overall, the work is soundly presented with solid data analyses and elaborations with clearly articulated methodologies and reasoning, combining both simulation and experimental approaches. It is indeed worthwhile publishing given the demonstrated significant progress built on the authors' prior work in Nature Nanotech. a few years ago. A few minor comments here: 1) Some more discussion over the overall strategies of the voltage control with respect to different pattern selection/determination could be helpful to the generalization of the new techniques besides the normal/current anodization approaches. 2) The electrical field simulation is well matched with experimental results, any experimental deviations that could be introduced due to various experimental factors? Or any examples of failures that could help shed some lights on the general practices? Some discussions along the line could be helpful to the adoption of the new technique here.
3) The demonstration of the out of plane shape variation through dynamic voltage control is well done, but how to fully utilize it as a template seems to be less elaborated, given the demonstrations are quite limited to the photon-responsive Yong Lei and his co-workers report a method to prepare complex yet high-fidelity nanoporous AAO templates by heterogeneous aluminium anodization rates with varied anodization voltages. Authors clearly introduced previous challenges and motivation for this work and demonstrated experimental evidence for their claim. As cited in ref. [22], authors previously reported a large-area nanofabrication of binary pored AAO templates in 2017. While there have been a number of applications of nanostructured AAO templates, reports on new strategy for nanofabrication has been limited. Hence, I enjoyed the reading this manuscript. In particular, I am impressed by the mix-and-match of different cross-sectional geometry for nanopores along the z-direction as well as spatial arrangements in x-y plane. Obviously, these new fabrication capabilities will contribute to overcome previous limitations and may open new horizons in various fields. Also, authors already performed extensive parameter studies and the experimental results are clearly supporting the main cliam. Hence, I am favorable for publication of this manuscript in Nature Communications.
However, considering free-form 3D nanoarchitectures produced by Nanoscribe, I think that "all-in-one platform" in the Abstract is overselling and ask authors to tone-down.

Point-to-point responses to reviewers' comments & description of the change we have made
to the manuscript to address these comments Re: Manuscript ID NCOMMS-21-32423-T We highly appreciate the detailed and constructive comments put forth by the reviewers, and have revised the manuscript accordingly, with all the reviewers' concerns being addressed. This letter details our pointby-point responses to the comments. The revised texts are written in blue font in our revised manuscript and supporting information.

Comments:
This is a very nice paper in which strategies for the three-dimensional tailoring of AAO membranes are comprehensively devised. Moreover, interesting applications of replicas of the three-dimensional AAO structures are reported. It is also apparent that the authors put a lot of efforts in this work. Therefore, I strongly recommend the publication of this work. Answer: We highly appreciate the reviewer for the comments.
1. There are some editorial points that could be improved. What is an "uneven anodization rate"? What is meant by "internally" and "externally bent walls"? Are concave and convex walls meant?
Answer: Many thanks for the comments. 1) "Uneven anodization rate" has been changed to "unequal anodization rate".
2) For better understanding, "internally" and "externally bent walls" are schematically illustrated in The revised statement about "internally" and "externally bent walls" on pages 3-4 of the revised manuscript: "the in-plane pore shape of the designable template can be continuously altered from polygons (e.g., triangle and square) with internally-bent (i.e., concave) walls to polygons with non-bent walls (i.e., straight) and then to polygons with externally-bent (i.e., convex) walls (please see Supplementary Fig. 1 for the schematic illustration of shape-different pores) 2. At first, it remains unclear and unmentioned that hard imprint lithography of the Al substrate to be anodized is the apparent starting point and that the topography of the surface of the stamps is a crucial feature for the following structure formation. This very crucial step needs to be highlighted in the introductionotherwise, it is hard to follow the manuscript.
Answer: Thanks for the comment. According to your suggestion, the "hard imprint lithography" and "topography of the surface of the stamps" have been highlighted in the introduction on page 3 of the revised manuscript: "Different from self-ordered two-step anodic anodization which only generates circular-shaped pores with close-packed honeycomb (i.e., trigonal) arrangement, artificially nanoengineering Al-foil surface by hard imprint lithography can guide anodic anodization to form pores with desired structural parameters (e.g., arrangement and interpore spacing) that highly depend on the surface topography of imprinting stamps"

The impact of the design of the imprint stamps and the role of the imprint step should be addressed.
Answer: We sincerely thank you for the suggestion.
The role of the imprint step is explained on page 4 of the revised manuscript: "Given that nanoimprinting Al-foil surface with appropriate texture could guide the initiation of pores 19 , we hope to generate unequal aluminium anodization rates by introducing uneven-profiled four-leaf cloverlike nanoconcaves onto surface (see layout of COMSOL simulation in Supplementary Fig. 2)" The impact of the design of the imprint stamps is explained on page 6 of the revised manuscript: "To discern the crucial role of inhomogeneous radial anodization rates (i.e., uneven EF distribution) in shape designability, we fabricated circular nanoconcaves for reference by using a Ni circular-pillar stamp (Supplementary Fig. 8) and achieved homogeneous radial anodization rate (as evidenced by even EF distribution) on the side-walls of nanoconcaves ( Supplementary Fig. 9d). Not surprisingly, the pore shape cannot be altered no matter what AV was applied ( Supplementary Fig. 10)." 4. The naming of the topographic features generated in the Al substrate as "nanodents" is somewhat misleading.
Answer: Thank you for the comment. The naming of the topographic features generated in the Al substrate has been changed to nanoconcaves. Figure 1  Answer: Thanks a lot for the comment. In the revised manuscript, we have further discussed (1) why we use constant-voltage anodization rather than constant-current anodization, (2) the appropriate AV range for pore-shape tuning regarding a specific array, (3) the appropriate AV range for a mixture arrangement.

The scheme integrated in
The reason that we use constant-voltage anodization rather than constant-current anodization is introduced on page 4 of the revised manuscript: "Here, we select potentiostatic anodization for structural controlling of pores in consideration of the linear relationship between AV and pore parameters (e.g., interpore distance and pore diameter) 17 " The appropriate AV range for pore-shape tuning regarding a specific array is described on page 7 of the revised manuscript: "It is found that concerning a specific arrangement, the adjustable AV for designing pore shape is limited in an AV range (denoted as appropriate AV range), out of which (i.e., in too-low AV and too-high AV ranges) the arrangement predetermined by nanoconcaves is broken (Supplementary Fig. 15). The three AV ranges are separated by two threshold values ( Supplementary Fig. 16), which are empirically observed to be V a and √3Va, where V a = 0.4 (V nm -1 ) × L h (nm) and L h is the interpore spacing of the hexagonal array (i.e., 400/√3 nm). In other words, AV thresholds can be derived from the linear spacing-AV relation, i.e., AV (V) = spacing (nm) × 0.4 (V nm -1 ) 24 , regarding two spacings (e.g., L h and √3Lh) of emerging arrays" The appropriate AV range for a mixture arrangement (i.e., spacing-different mixture arrangement, mixture of tetragonal and octagonal arrangements) is discussed on page 9 of the revised manuscript: "we hypothesized that AVs of a spacing-different mixture arrangement should lie in an intersection of several appropriate AV ranges to simultaneously prevent the occurrence of new pores and the disappearance of existing pores for every constituent arrangement. To test this hypothesis, we mixed the above tetragonal arrangement with spacing of L t (i.e., 400 nm) with a larger-spacing ( √2 L t ) tetragonal arrangement ( Supplementary Fig. 22, a to c). In theory, the appropriate AV ranges for two arrangements are (V b /√2, √2Vb) and (V b , 2V b ) (see Supplementary Fig. 22, d to f for details), where V b = 0.4 (V nm -1 ) × L t (nm). 24 The intersectional AVs locate from V b (160 V) to √2Vb (~226 V). " "Regarding the octagonal arrangement with an appropriate AV range from √2.5Vb to √5Vb (please refer to Firstly, we decorated the Al-foil surface with circular nanoconcaves to investigate the role of the preset shape of nanoconcanves. We found that the anodized pores had circular shape and were not alterable with the applied AV (Supplementary Figure 10). We then carried out electric field simulation to shed light on the underlying mechanism. In Supplementary Figure 9, the circular-shaped nanoconcaves demonstrate homogeneous radial anodization rate. Based on these results, we conclude that the uneven profile of the four-leaf clover-like nanoconcaves that cause inhomogeneous radial anodization rates plays a critical role in shape designability.
The importance of preset shape of nanoconcaves on Al-foil surfaces is described on page 6 of the revised manuscript: "To discern the crucial role of inhomogeneous radial anodization rates (i.e., uneven EF distribution) in shape designability, we fabricated circular nanoconcaves for reference by using a Ni circular-pillar stamp ( Supplementary Fig. 8) and achieved homogeneous radial anodization rate (as evidenced by even EF distribution) on the side-walls of nanoconcaves ( Supplementary Fig. 9d). Not surprisingly, the pore shape cannot be altered no matter what AV was applied ( Supplementary Fig. 10)" As for the anodization electrolyte, we found that as the AV was much higher than the value following the linear space-AV relation (DOI:10.1021/nl025537k), the AAO templates tended to suffer from electrolytic breakdown. To prevent its occurrence, we introduced organic solvent to slow down anodic anodization and meanwhile decreased the H 3 PO 4 concentration of anodization electrolyte. The importance of anodization electrolyte is introduced on page 18 of the revised manuscript: "Regarding aluminium foils patterned with nanoconcaves of tetragonal arrangement and 400 nm spacing, the anodization electrolytes were selected on the base of AVs. That is because higher AVs accelerate Al anodization (i.e., high anodization current) and result in accumulation of heat (and temperature increasing), which causes electrolytic breakdown of AAO templates and prevents the formation of pores with high aspect ratios. Therefore, as AVs were lower than 180 V, anodization was conducted in 0.4 M H 3 PO 4 solutions; while beyond 180 V, a mixture solution including 3 mL H 3 PO 4 , 300 ml ethylene glycol, and , and 600 ml DI water was exploited as anodization electrolyte. The presence of ethylene glycol and the reduction of H 3 PO 4 concentration can effectively mitigate electrolytic breakdown of AAO template at high AVs." 3) The demonstration of the out of plane shape variation through dynamic voltage control is well done, but how to fully utilize it as a template seems to be less elaborated, given the demonstrations are quite limited to the photon-responsive examples. Any other potentially carriers?
Answer: Many thanks for the comment.
As indicated in the introduction of the revised manuscript "nanostructures are subject to physical and chemical property variation as a function of their geometry and composition", the out-of-plane shape- Moreover, as mentioned by the Reviewer 3 in the comment (3) "Stimuli-responsive micro/nanostructures are based on non-uniform responses upon external stimuli. Thus, this work will have a great potential in that field", we also think that nanostructures with out-of-plane shape variation (i.e., non-uniform in size, shape, and/or composition along the axial direction) are capable of generating non-uniform responses as applying external stimuli, thus owning "potential applications in stimuli-responsive systems".
All the above-mentioned points demonstrate the high application potentials of the out-of-plane controllable nanostructures. However, considering the space limitation of a paper, here we took the optical application as an example to show the advantages of out-of-plane well-defined nanostructures. In the future, we will definitely focus on the application of these out-of-plane controllable nanostructures in different fields. Answer: Many thanks for the nice comments.

1) However, considering free-form 3D nanoarchitectures produced by Nanoscribe, I think that "all-in-one
platform" in the Abstract is overselling and ask authors to tone-down.
Answer: Thanks for the suggestion. We have revised it on page 1 of the revised manuscript: "In light of the broad selectivity and high controllability, designable templates will provide a useful platform for well-defined nanostructuring." 2) While it's for microstructures and based-on a growth mechanism, below reference is relevant and worthwhile to cite in the Introduction as an effort to prepare uneven cross-sectional shapes for small-scale fabrication. https://www.nature.com/articles/ncomms7584 Answer: Thanks a lot for your kind advice. We have cited this important article as Ref. 12 and properly mentioned in the introduction on page 2 of the revised manuscript: "Various nanostructuring techniques have been developed, such as photo/electron-beam lithography, selfassembly, nanoimprinting, and template-based techniques 11 as well as material growth controlling 12 , while almost none of these techniques fulfills all the above six capabilities of well-defined"

3) Stimuli-responsive micro/nanostructures are based-on non-uniform responses upon external stimuli.
Thus, this work will have a great potential in that field. Below reference is worthwhile to cite in the "Due to the compelling requirement of device miniaturization, synthesis of nanoscopic structures and their macroscopic integration into a large-scale array are fundamental to modern and future devices in the fields of optics 1 , electronics 2 , telecommunication 3 , biology 4 , energy conversion/storage 5,6 , and stimuli-responsive materials 7 , etc." Potential application of well-defined nanostructuring in stimuli-responsive system is highlight on pages 12-13 of the revised manuscript: "These well-defined nanostructures with high freedom of structural designability could result in some unique properties. Furthermore, external stimuli (e.g., capillary force, light, magnetic field, and heat) will further adjust these properties in a dynamic way. 7 "